onDocumentUpdate event is typed as having data potentially undefined

It seems weird that an update event could contain nothing, if that's the case an update event did not occur, did it ?

```ts
// firestore.d.ts
export declare function onDocumentUpdated(document: Document, handler: (event: FirestoreEvent | undefined, ParamsOf>) => any | Promise): CloudFunction | undefined, ParamsOf>>;
```

It is annoying because it forces an additional, seemingly unnecessary, check.

Start with the firestore.d.ts declaration shown in the issue and inspect the onDocumentUpdated event type. Confirm the intended non-optional event type and verify that the generated declaration no longer requires an unnecessary undefined check.

索引モデルが issue の本文から書いたものです。